In many engines with fuel injection systems, it is desirable to supply liquid fuel to the injector or injectors at a pressure which varies as a function of the manifold pressure so that the pressure drop across the injectors remains constant. The manifold pressure and the flow rate of fuel supplied by the injectors to the engine each vary with engine speed, load and other operating conditions.
Previous fuel supply systems have been developed, one of which is shown and described in U.S. Pat. No. 5,148,792. This system has a fuel tank with a fuel pump to supply fuel under pressure through a fuel line to a fuel rail coupled to a fuel injector for supplying fuel to the engine cylinder. The pump includes a pressure sensor which provides an electrical signal as a function of fuel pressure at the pump outlet to an electronic control to vary the speed of the pump to maintain a constant pressure within the fuel line. However, this system cannot maintain a constant pressure differential across the injectors.
Previous systems have been known to include a pressure regulator which has a manifold reference to maintain a constant pressure drop across the injectors. One such regulator is disclosed in U.S. Pat. No. 5,265,644. However, these prior regulators cannot accommodate any increase in pressure caused by fuel expansion due to heat rise and do not accumulate the increased volume of the heated fuel. For example, during engine deceleration the injectors may close trapping fuel in the fuel rail. The high temperature within the fuel rail causes the fuel to be heated and expand which increases the pressure in the fuel rail.
Pressure rise and fuel expansion in the rail also occurs during conditions known as hot soak. Hot soak conditions occur when the engine has been idling or running at slow speeds especially during hot weather or when the hot engine is turned off. The high temperature in the fuel rail plus the hot ambient air causes the fuel trapped in the fuel rail to be heated and expand. Some pressure increase is desirable to prevent fuel vapor formation. However, excessive pressure in the fuel rail is undesirable since it could force fuel through the injectors causing leakage and/or malfunctions.
In these by-pass type regulators, any fuel pressure above the set system pressure is relieved by returning fuel to the tank through a fuel return line. Accordingly, these devices maintain only a set system pressure. In addition the returning fuel may have an elevated temperature which may cause unwanted vaporization.